1. Field of the Invention
The invention relates to fly wheels for energy storage and methods and apparatus for making such flywheels. More particularly, the invention relates to a fiber composite flywheel and a method and apparatus for making it wherein the fiber composite flywheel includes a combination of spiral fibers in the hoop direction with radial fibers in order to optimize strength and stress properties.
2. Brief Description of the Prior Art
A flywheel is a device for storing energy or momentum in a rotating mass. Large engines require a heavy rotating mass to carry them smoothly through pauses between jerky piston strokes. Automobile engines today have smaller flywheels that perform the same essential task of storing energy. Flywheels convert electrical energy to kinetic energy, and the more energy put into the flywheel, the faster it spins. Increasing a flywheel""s speed increases its energy density, or the amount of energy that can be stored and recovered per unit weight; thereby increasing the efficiency and cost effectiveness of the flywheel.
The strength to density ratio of the material used in a flywheel rotor is critical. Carbon fiber composites are preferred for high performance flywheels because of their high strength and low density. A denser material has more moss packed into a given volume and kinetic energy for a given speed increases with mass. Steel is denser than carbon fibers, so steel molecules have more energy than fiber molecules when they are moving at the same speed.
To maximize the energy storage of a flywheel, it is generally desirable to maximize the ratio of strength (s) to density (r). This ratio (s/r) makes it easy to compare the energy storing potential of different materials. For steel, s/r is at most 966,000. Carbon fibers have a higher tensile strength, up to 1 million psi, and a much lower density, around 0.06 lbs/in3. For these fibers, s/r is 17 million, about 17 times better than steel. This means that although carbon fiber is only 3.6 times stronger than steel, it can store almost 17 times more energy per pound.
Another advantage of fiber-reinforced composite rotors, as compared to metallic rotors, is that they have been shown to fail in a less destructive manner than metallic rotorsxe2x80x94an important factor for safety reasons. Composites offer benign failure modes, which must be considered when addressing flywheels. Other flywheel materials such as steel or ceramics fail by rupturing into large high-energy fragments, which must be contained with shielded canisters. Composites, on the other hand, fail by radial delamination or disintegration, thereby resulting in ejection of short fibers and small matrix particles. This potentially allows for lighter containment vessels to ensure their safety.
One method for making composite flywheels is the cross-ply method. A cross-ply composite flywheel is made from circular shapes cut from a flat woven material. Each layer of material is laid down differently, so that the angles of the fibers are different relative to one another, in order to simulate an isotropic material, like steel. The resulting flywheels are less than ideal. An example of the cross-ply method is shown in U.S. Pat. No. 4,102,221.
Another method for making composite flywheels is filament winding. The filament winding process takes bundles of fibers, dips the fibers in a resin bath, then continuously winds the flywheel from the inner diameter to the outer diameter with larger and larger concentric rings of circumferential fibers. The advantage of filament winding is the speed of manufacture. The disadvantage of a filament winding system is the lack of radial fibers in the final product. Filament wound flywheels with no radial reinforcements fail prematurely as the radial stresses encountered in use create delaminations between the concentric rings of circumferential fibers. The individual rings of fiber stretch at different rates, opening space between the rings and stressing the epoxy filler. U.S. Pat. No. 4,285,251 shows an example of a filament winding process.
Another method for the manufacture of composite flywheels is polar weaving. Polar weaving involves the interlacing of two sets of yams, warp and weft, and using a special take-up system to advance the outer diameter yarns at a higher rate than the inner diameter yams to form a circle. The take-up rollers are changed from a straight cylinder to a conical shape. The result is a spiral woven material. When continuously-woven, the plies of material will spiral on top of each other to create a disk.
The primary disadvantages of the polar weaving process are that it is a slow process and the interlacing of the yarns creates crimp in the fiber, which reduces the tenacity (strength) of the fiber and reduces the maximum fiber volume in the flywheel. Crimp reduces the mechanical properties of the fiber because the molecules in the fiber are no longer aligned with the stresses. Also, when the fibers are crimped, the mechanical properties are drastically reduced, particularly when the material is fatigued. U.S. Pat. No. 6,029,350 provides an example of the polar weaving process.
U.S. Pat. No. 5,778,736 describes a fiber composite flywheel having hoop fibers and radial fibers interwoven with the hoop fibers. However, the flywheel design of this patent suffers from the disadvantage that the interwoven radial fibers are crimped in the manufacturing process thereby detracting from their physical and mechanical properties.
In a first aspect, the present invention relates to an apparatus for the manufacture of composite flywheels. The apparatus includes an inner cylinder, an outer cylinder, a fiber feeding device and a knitting device. The fiber feeding device includes at least a set of conical feed rollers for feeding hoop fibers to the cylinders. The knitting device employs a circular knitting machine fitted with a radial fiber guide for feeding the radial fibers to the cylinders. The apparatus also includes structure for raising and lowering the needles of the knitting machine at the proper times to effectively feed the radial fibers to the cylinders. The fiber feeding device and knitting device interact to provide layers of hoop and radial fibers to the cylinders to form a flywheel pre-form including a combination of radial and hoop fibers without introducing crimp into the fibers.
In a second aspect, the present invention relates to a method for the manufacture of composite flywheels. In the method, a coil of spiral fibers is made in the form of a pre-form. During the step of making the coil of spiral fibers, radial fibers are interleaved with the spiral fibers to provide additional reinforcement in the radial direction. The radial fibers are interleaved as a layer between successive layers of hoop fibers to avoid introducing crimp into the radial and/or hoop fibers during the manufacturing process.
In a third aspect, the present invention relates to a fiber composite flywheel having excellent physical properties. More particularly, the fiber composite flywheel of the present invention includes a coil of spiral fibers interleaved with radial fibers to provide excellent strength and stress properties. One advantage of the invention is that the radial fibers are interleaved with the hoop fibers without crimp in the radial fibers thereby avoiding the disadvantages associated with fiber crimp.